1. Field of the Invention
This invention relates to an electrophotographic photosensitive member, and, more particularly, it is concerned with an electrophotographic photosensitive member which is less likely to fatigue after repeated use at a high speed.
2. Description of the Prior Art
For an electrophotographic photosensitive member, many embodiments have been employed in accordance with the specific electrophotographic process to be applied. Of these various electrophotographic photosensitive members, there is one having an insulating layer on its surface which forms an electrostatic latent image on the insulating layer. An electric charge is required to be injected in the interface between the insulating layer and the photoconductive layer by electric charging. As one example of such electrophotographic process, there is a process comprising a primary charging, image exposure, AC charge removal simultaneously with or subsequent to, the image exposure, or alternatively electric charging in the opposite polarity to that of the primary charging, and then whole surface exposure. When the photoconductive layer is made of a p-type semiconductor such as Se, SeTe, etc., the primary charging is conducted by a negative corona discharge to inject a positive charge from the supporting body, or substrate, into the photoconductive layer, and the electric charge is caused to migrate to the interface between the insulating layer and the photoconductive layer in accordance with the electric field applied to the photoconductive layer. When it is difficult to inject the charge from the substrate, uniform light irradiation is effected onto the photosensitive member immediately before, or simultaneously with, the negative corona discharge, whereby an appropriate quantity of the positive charge can be provided to the interface between the insulating layer and the photoconductive layer. when this light irradiation is conducted from the side of the substrate, the substrate should be made of a light transmitting material such as, for example, nesa glass, light transmitting resin film, and so on. When the photoconductive layer is constructed with an n-type semiconductor, the charge polarity in the primary charging is positive, and the charge migrating to the interface between the insulating layer and the photoconductive layer is negative. At the initial charging, the injection of an appropriate quantity of electric charge between the insulating layer and the photoconductive layer is indispensable for producing an electrostatic latent image of high contrast. For this purpose, when the electrically conductive substrate is made of, for example, a metal material, a charge injection layer is provided between the substrate and the photoconductive layer as disclosed in Japanese Patent Publication No. 49-6223 (corresponding to British Patent Specification No. 1,335,699). This charge injection layer supplies sufficient electric charge to the photoconductive layer at the time of the charging, and thereby contributes to the presence of an appropriate quantity of charge between the insulating layer and the photoconductive layer. Even by addition of the charge injection layer, however, there still exists room for improvement. That is, when the photosensitive member is repeatedly used and, in particular, when its cycle of repeated use is accelerated, the charge quantity present in the interface between the insulating layer and the photoconductive layer is gradually reduced. As a result it is often observed that the electric potential at the dark portion of the electrostatic latent image after repeated operations, approaches the potential of the bright portion thereof in comparison with its initial level, and the image contrast is lowered. This has been explained as the so-called "fatigue phenomenon" of the photosensitive member.